1. Field of the Invention
The present invention relates to an image pickup system for endoscope which radiates illuminating light to a subject existing at a dark place to perform contrast AF, and a method for operating the image pickup system for endoscope.
2. Description of the Related Art
As ranging methods for acquiring information about a distance to a subject, an active method in which infrared, ultrasound or the like is radiated to the subject, and a distance is detected based on a time period required for a reflected wave to return or a radiation angle, and a passive method in which ranging is performed with use of an image acquired via an optical system without using infrared or the like are known.
Examples of the passive method of the methods include phase-difference AF (AF: autofocus) used in single-lens reflex cameras and the like and contrast AF widely used in video cameras, compact-type cameras and the like.
The phase-difference AF is not so often used in small-sized image pickup apparatuses because, though focusing speed is relatively high, the phase-difference AF utilizes a luminous flux which has passed through an objective lens and, therefore, for example, such a configuration is adopted that the luminous flux is caused to branch to a phase-difference AF sensor side, the phase-difference AF sensor being different from an image pickup section which picks up an image.
On the other hand, the contrast AF does not require a separate AF sensor or the like because the contrast AF utilizes an image picked up by the image pickup section to perform AF. Therefore, the contrast AF is adopted in small-sized image pickup apparatuses. In order to determine a focus position (a peak position of an AF evaluation value (contrast or the like)) in the contrast AF, it becomes necessary to perform scanning for acquiring a plurality of (generally, three or more) frame images while causing a position of a focus lens to change. Especially, in a case of scanning a whole scanning range, a whole range within which the focus lens can be driven, the number of acquired images increases, and a long photographing time period is required.
Thus, it has been proposed to reduce the number of frame images required before being focused in the contrast AF.
For example, Japanese Patent Application Laid-Open Publication No. 2008-111897 describes an example of an operation in a mode in which a wire net or a cage existing in a vicinity is prevented from being focused, and describes a technique in which a distance to the wire net or the like is measured based on an output of a line sensor which performs phase-difference AF, a vicinity of the measured distance to the wire net or the like is excluded as a ranging range, and contrast AF is performed in a ranging range other than the ranging range while a position of a focus lens is being changed.
Further, Japanese Patent Application Laid-Open Publication No. 2009-133903 describes a technique in which an predicted focus position is calculated based on a size of a target object detected by a target object detecting section (such as a face detecting section), and contrast AF is performed with a vicinity of the calculated predicted focus position as a start point of movement of a focus lens.
Both of the techniques described in the respective patent gazettes aim to shorten a focusing time period by, when performing contrast AF, acquiring a distance to an object by performing auxiliary measurement to eliminate necessity of causing the focus lens to move in the whole scanning range.